| dc.description.abstract | Nitrogen (N) and Phosphorus (P) are critical elements required by organisms for cellular processes and are the key limiting nutrients in most terrestrial and aquatic ecosystems. However, massive nutrients have entered the biosphere over the last century, largely through fertilizer use, fossil fuel emissions, and wastewater discharge, substantially perturbing global N and P cycles. Most anthropogenic nutrients are applied on land and can be transported to coastal oceans through the land-river-ocean continuum. Nutrient enrichment in aquatic ecosystems has resulted in a cascading set of environmental problems, such as harmful algal blooms, hypoxia, ocean acidification, and fish-killing. Understanding nutrient dynamics along the land-ocean continuum is essential for reducing eutrophication and protecting the drinking-water supply. In this dissertation, we developed an anthropogenic nutrient input dataset and connected nutrient cycles across terrestrial and aquatic ecosystems in a process-based model framework. The model combined with nutrient input data was applied to quantify N and P dynamics along the land-ocean continuum and comprehensively investigate the impacts of climate and human activities on nutrient loading.
The biogeochemical processes of riverine N and P dynamics and a water transport module were incorporated into a process-based biogeochemical model, Dynamic Land Ecosystem Model (DLEM). The N and P cycles across terrestrial and aquatic ecosystems were coupled within the modeling framework. The model is capable of simulating riverine exports of four N species: ammonium (NH4+), nitrate (NO3–), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON), as well as four P species: dissolved inorganic phosphorus (DIP), dissolved organic phosphorus (DOP), particulate organic phosphorus (POP), and particulate inorganic phosphorus (PIP).
Livestock manure nutrient is one of the major anthropogenic nutrient sources for agricultural systems. There is still a lack of spatially-explicit estimates of manure nutrient input datasets over a century-long period in the United States (U.S.). We developed datasets of livestock manure N and P production and application in the contiguous U.S. at a 30 arc-second resolution during 1860-2017. The total production of manure N and P increased over the study period driven by increased livestock numbers before the 1980s and enhanced livestock weights after the 1980s. The intense-application region enlarging from the Midwest toward the Southern U.S. may bring high risks to water quality in coastal oceans.
The improved DLEM was applied to assess the long-term dynamics of P loading from a typical agricultural basin, the Mississippi River Basin. Simulations showed that riverine exports of DIP, DOP, POP, and PIP increased by 42%, 33%, 59%, and 54%, respectively, from the 1900s to the 2010s. Riverine DIP and PIP exports were the dominant components of the total P flux. DIP export was mainly enhanced by the growing mineral P fertilizer use in croplands, while increased PIP and POP exports resulted from the intensified soil erosion due to increased precipitation.
The N and P budgets were quantified to evaluate legacy soil nutrients across the Mississippi River Basin based on DLEM and long-term anthropogenic nutrient inputs. Both N and P balances, the difference between nutrient sources and sinks, increased substantially from 1930 to 1980, then P balance decreased dramatically within the 1980s while N balance showed a slight decline. The enhanced crop nutrient use efficiency decreased nutrient surplus and slowed down the accumulation of legacy soil nutrients. The longer resident time of P in soils leads to a much slower release of soil P to rivers than that of N. The increasingly dominant role of legacy nutrients as loading source combined with enhanced extreme precipitation may contribute to the decreasing N:P loading ratio.
This dissertation filled the knowledge gap of synthetically evaluating dynamics of different nutrient species across the land-ocean interface under the impacts of multiple environmental changes over a century-long time scale and large spatial scale. The results can shed light on nutrient management and water pollution control strategies for the Mississippi River Basin, the Gulf of Mexico, and other similar regions. | en_US |
